1. Field of the Invention
The present invention relates to a bleed arrangement for gas turbine engines.
2. Description of the Prior Art
More specifically, the bleed arrangement is of the type described in U.S. Pat. No. 3,360,189, issued Dec. 26, 1967, D. L. Cook, inventor. In that patent, a piston having opposed faces is subjected to air pressure on opposed faces thereof. One of the piston faces which is flat is adapted to cooperate with a valve seat peripherally of an exit port intermediate the compressor. The piston is meant to throttle the bleed air from the compressor.
In present bleed valve configurations as well as those described in U.S. Pat. No. 3,360,189, there is provided a flat radially extending valve seat adapted to receive flat radial piston surfaces, and as the piston is closing on the valve seat, a peculiar phenomenon occurs. The piston begins to chatter or vibrate, thus causing a loss of bleed control. The space between the piston and the valve seat represents a nozzle. According to Technische Stromungslehre, Von Brand Eck, page 28, the phenomenon is described as being based on the fact that the inside diameter of the so-formed nozzle is of a smaller diameter than the outside diameter; therefore, the nozzle area is increasing diametrically outwardly, thus creating a diffuser passage. Since the outside pressure is atmospheric, then the pressure inwardly of the nozzle is below atmospheric creating suction areas, thereby rendering the piston unstable just before it is completely closed with the valve seat.